GN: Don't build tests by default.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index b1301b01650b63fef076082160aa0b6b71635f63..9d98ca181c68e6e04b6c6e28aa91bda1782363fa 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -6,12 +6,18 @@
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
+import("webrtc/build/webrtc.gni")
+
 group("default") {
   testonly = true
   deps = [
     "//webrtc",
-    "//webrtc:webrtc_tests",
-    "//webrtc/examples",
-    "//webrtc/tools",
   ]
+  if (rtc_include_tests) {
+    deps += [
+      "//webrtc:webrtc_tests",
+      "//webrtc/examples",
+      "//webrtc/tools",
+    ]
+  }
 }